Markov

I have refrained from commenting for awhile as i don’t want to discourage folks from experimentation or cause Sparkfun any grief but the fact of the matter is that a PID controller is typically used for applications such as this. You can get a PID controller with 64 ramp functions (so you can easily preprogram most any soldering profile) that will accept input from most any type thermocouple that also has RS485 capability for $97 from a prominent engineering supply firm in the US known for superb technical support. They run from $35 on up with less features on the web.
You could program the Spark Fun controller to act as a PID controller with a lot of time if you are really good at math. The basic equations are rather well published on the web and in the academic literature (which should warn you that it can become mind boggling enough to be a regular research subject).
PID stands for Proportional–Integral-Derivative control. You can program your profile using simple button menus on the face or with RS485 and optionally with a USB 485 adapter ($99) on the computer end in order to control this via a computer using free software (which is typically supplied as free in the industry). You will be using half-duplex RS-485 as PIDs generally hook up with two terminals (this involves using just one twisted pair of wires say in a typical cat5 ethernet cable adapted to the purpose (the above USB adapter comes with 6 feet of cable and has a typical jack in the end of what looks like a typical USB bluetooth or wireless adapter otherwise).
What a PID does is incorporate 3 types of controls–that is besides a the simple so many degrees a minute proportional control to whatever setpoint–it also monitors the rate of heat up and anticipates the approach to a setpoint. They typically incorporate some sort of “autotune” feature whereby it can learn from its own experience (both derivatives and intergrals). I have run into some claiming that their better models for sale include “fuzzy logic”, which i find humorous.
The rate of heat up for example is a simple calculus calculation of the first derivative of a function–the slope of the function(if you are really good at math, then this is simple intro calculus relative to vector analysis and other advanced calc courses you could chose to endure).
The other advantage of a PID over the home-made controller is that the PID contains the cold reference junction for a thermocouple which means that you can attach a thermocouple to a trace or pin or just an area of the PCB to get a more meaningful temperatures in your oven (creating a cold reference junction yourself is a rather advanced thing to do if you are not a whiz at electronics and math). Typically used are .005" Type K thermocouples that can be soldered in place using high temperature solder or else are taped (most common) or glued into place. Bare wire thermocouples are sometimes used which are rather inexpensive–you need to insulate these yourself generally with high temp fiberglass/teflon (PTFE) braid. For slightly more you can get premade thermocouples with 3 to 6 foot leads attached–some with mini plugs and wind up reels. The thinner the wire, the quicker the response time–which means a more accurate process. Sizes down to .0005 are sometimes used but these are rare and very fragile (i only found 0.0005" as bare wires with 8 inch leads).
The relay used should not be a coil relay as the response time is just not up to par. You should be using a Solid State Relay (SSR)–there are eons of options for say 3-32 volt input and AC output. A class 6 relay will have “finger protection” but still should be enclosed in a box/case or inside the oven’s control “box”. Some have finger protection covers as options. A good relay will set you back at least $40.00–you pretty much get what you pay for and there are loads of cheaper versions everywhere that hobbyists tend to use (too often i figure). Cheaper relays tend to have slower response times and to be a bit sloppy otherwise.
One warning about hooking up your computer to a controller operating a relay–you could technically fry your computer should the relay fail in certain ways and given most are resorting to cheap import relay models—well–use the old junk laptop. Some RS-485 master controllers and adapters are optically isolated as are some relays (i am resorting to an optically isolated class 6 SSR) which tend to offer protection from the occurence.
You can get by just fine with an oven, relay, and your watch. You can get by just fine with the Spark Fun controller. You do have another option that can arrive very close to a professional reflow oven–the only difference is that you will be at the lower end of the preferred ramp rate as your toaster oven won’t be capable of up to 6 degrees C a minute and that when valuable production runs are set up–they actually do a lot of experimentation to determine the best profile for that specific run in advance and use multiple thermocouples to measure temperatures here and there including specific vulnerable ICs themselves.
Just to put things in perspective. If you need a good tool and want to get to soldering i would advise going the commercial PID/thermocouple route rather than a “home-made” controller that would involve extensive complicated programming to be any where near par. If you need a project to keep you busy and challenge your programming mind–the Spark Fun controller might be worthy of your own talents.
(there are even more complicated routes available for a little more money such as a device to read say 8 thermocouples which likely would involve a separate master controller and an adapter for the computer that might be more apt if you are seriously going to do production runs of if you are really just into this sort of thing). I think there is maybe better science/math involved for the inquisitive kid going the PID route also.

There are a few misconceptions regarding the health risk of solder containing lead. My understanding is: that lead will not produce an airborne hazard at soldering temperatures (when it melts) it needs to be raised to very high temperatures before it volatizes–produces an airborne hazard. That is not to say you don’t need adequate ventilation. The fluxes used do produce irritant fumes.
Lead can on the other hand can be absorbed through the skin. just as it is in the lungs. Artists still use some lead-based pigments–the real danger is in their powder form which can be easily inhaled and some do use these to blend their own paints. (most avoid the powders settling for paste forms which are more easily controlled–those using powders tend to be well-schooled so they say. These pigments such as white lead are not available in hobby art supply stores). The point is that lead containing pastes, while hazardous are relatively safe in that exposure can easily be safeguarded with reasonable use.
It has been said that the personal exposure hazards (from fumes) can be greater from some versions of non-leaded solders. So using non-leaded solders does not put your health-risk at ease. You still need adequate ventilation. I learned this after my own reactions to exposure to non-leaded solder were somewhat severe (plumbing rather than electrical components. A plumber did a little soldering in the house and i had a reaction just by being in the house to the fumes from a zinc chloride flux). Non-leaded solder has some rather nasty substances in itself. So do all fluxes.
The main concern about lead solder is to reduce the footprint of lead in our lives as there is no telling where it eventually ends up.
The main risk to having solder paste that contains lead in the house is the fact that it is advised to put it in the fridge–THIS IS ENTIRELY DANGEROUS ESPECIALLY IF ONE HAS LITTLE KIDS. IN NOW WAY SHOULD ONE STORE THIS IN THE SAME FRIDGE IN WHICH YOU STORE YOUR FOOD. No telling who might make a unwitting mistake including yourself in some sort of stupor (it happens) and what residues are on the outside of the package that could transfer to foodstuffs. You should have a small fridge for the purpose in your shop with a hasp lock on it of some kind.
The other real hazard is the little bits and pieces that can be left behind after its use which can be ingested or cause other exposure by accident–either kids or adults. Also q-tips and what ever used to clean up paste residues can pick up lead residues. You don’t want lead particles to become loose in your house or to ever solder on a table you might eat off of or in the kitchen or other ‘living areas’ would be my advice.
Else, from a societal standpoint the problem is the left overs that end up in the waste stream and that the product itself which bears lead in its solder eventually reaches the trash stream also creating a problem for landfills. I would expect that eventually lead solder will not be obtainable at all.
Having said that, lead solder does its thing at a temperature substantially less than that of lead-free solder. Lead-free solder temperatures involve a lot more risk of damaging components–which is not easy to detect or verify as its consequences may not immediately materialize. Hand soldering (or toaster oven or skillet conversions) are less controllable lending significant technical advantages to using lead solder for prototyping and personal production.
I myself don’t have a great liver and have a severe autoimmune problem (no not AIDS, i have a genetic condition–Addisons) and have always been extremely chemically sensitive. I use the nitrile gloves to handle solder ‘wire’ and i generally solder in the garage where i have installed a very beefy fan in the side door–so i set up a work table in front of the fan. I also wear a filter mask with cartridges that are effective at removing flux fumes. If you do a lot of soldering i would recommend you consider your exposure to any sort of soldering as serious. The problem is that most of the risk is very long-term and while one is young, one tends to procrastinate. You can mature to resent your youthful folly.
The short of it is that solder containing lead is not going to risk your health if handled wisely. That using lead-free solder unwisely also introduces significant health risks. The trend toward non-leaded solder is driven by larger societal concerns about reducing the amount of lead in products that are hard to account for–such as byproduct wastes of production and eventually that everything finds its way into the waste stream. Otherwise, being of a general hazard leaving no real trail, it can end up who knows where in our homes and other environs.
A major source of lead exposure has been plumbing. It can sometimes be a matter of the solder that was used profusely once upon a time (now copper is too expensive for routine use). But when i was young i witnessed how plumbers tended to seal threaded pipes. Many of them used to buy a gallon of leaded paint, which was common, and decant it (pour it off before mixing) which left the leaded component behind in the bottom of the can–1 to 2 inches. They then would dip the male ends of threaded joints into this leaded concoction to seal piping for heating systems and sometimes potable water supplies–it was cheaper than commercial pipe dopes and most used left over paints from construction i would guess.

does anybody have a favorite toaster oven?
The controller is interesting but i just ordered a programmable 64 ramp PID tailored for furnaces, 3 foot K type oven thermocouple, 25 amp DC in (3-32) AC out relay and heat sink for control. I think the usual application for the PID is ceramic kilns which generally follow a ramped temperature profile working their way up in steps to a final temp. Comparable cost–maybe less.
A better application for an arduino or like controller i think would be an application to monitor a glass high temperature thermistor snugged onto a piece of junk PCB stock to help ascertain what a PCB experiences in order to collect data useful for tweaking the profile ramped into the controller? Shouldn’t be too hard. Found this NTC Thermistor good for -50 to +300 degrees Centrigrade i recall:
GC4914A-3-100
data sheet: http://www.specsensors.com/pdfs/ntc_radial_glass.pdf

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